Oh, you're welcome!
I kind of do systematic checkover when I buy Livebearers. I'll post what I do and what I look for...feel free to ignore it though!
Body: Any healthy live-bearer's abdomen should look convex (slightly), when viewed in a profile. They should appear well-fed. Be careful of fish with protuding scales, and bloated bellies! Most times, these fish are incurable. A large head, that looks disproportionate to it's body, is usually a sign the fish has undergone long hunger periods. Doesn't mean it will die, but I would not reccommend buying him..
Gills: Healthy fish breathe slowly, although they breathe somewhat faster in a densely populated tank. As the fish breathe, they expose the gill cover. This should appear a pale reddish color, in healthy fish.
Fins: Missing or partially developed fins are ALWAYS a bad signal. Frayed edges on the fin, are usually the sign of a disease, although there may be other causes (Liiiike...nipping from other Guppies!) In a healthy fish, the fins should visably stick out from the body. A fish that clamps it's fins, and maybe rocks from side to side is definatly not feeling well, and may easily get a disease.
Eyes: The eyes should appear clear, and never protrude un-natuarally from the head.
Skin: The skin should not show any white spots, mold, fungus, or white film. The scales should hug the body, and look smooth. Be sure to examine the tip of the mouth with extra care, to be sure there is no white film.